Last year’s record canola harvest of nearly 18 million tonnes was a significant achievement for Canadian producers and industry stakeholders, far surpassing the goal set by the sector of 15 million tonnes by 2015. On the heels of this accomplishment, canola disease experts warn blackleg could take a large bite out of future production if the durability of current resistant varieties isn’t protected.
Before blackleg-resistant varieties entered the market in the mid-90s, farmers reported canola yield losses of up to 50 per cent. Since then, largely due to resistant varieties, blackleg has been kept to trace levels. However, over the past few years, survey results indicate an increase in disease incidence. This rising trend is a result of improvements to disease detection, changes in the pathogen itself and existing management practices, says Clint Jurke, an agronomy specialist with the Canola Council of Canada. Increased levels of blackleg in fields negatively affect yield and quality of crops, but they also hinder access to international markets.
Unless measures are taken to protect current commercial resistant varieties, producers could lose the one tool that has kept the disease at bay. Genetic resistance can be overcome by new virulent races of the blackleg pathogen. “This is a pathogen with high evolutionary potential that can overcome resistance fairly quickly,” says Jurke.
“Unfortunately, when we’re dealing with blackleg, we’re dealing with the worst-case scenario — it’s a pathogen that has high gene flow and high pathogen diversity. In order to best manage blackleg resistance so that it is durable into the future, you need to have a fairly complicated strategy in place using major resistance genes, stacking that with quantitative resistance and deploying that on some type of regional basis. We’re not doing that right now in canola. We are doing a fairly good job at having some major resistance genes and quantitative resistance, but I think there is a need for more work to ensure resistance is durable in the long term.”
Uncovering the genes
Uncovering the specific genes in a variety providing resistance to blackleg is an important step toward managing the disease and prolonging that resistance. According to Jurke, work of this nature in Canada is presently being led by Dilantha Fernando, a professor of plant pathology at the University of Manitoba. Researchers are determining and characterizing the blackleg resistance genes of commercial canola varieties.
“There are quite a few varieties that contain multiple types of resistance and some only contain one resistance gene,” says Jurke. “Half of all the canola varieties have a single resistance gene called Rlm3 as well as some undetermined background resistance… There is potentially a concern for the industry that we are relying for the most part on one type of resistance.”
In order to protect that resistance, says Jurke, the resistance genes must be stacked in different ways. “Currently, that’s not the way our industry is working,” he says.
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Characterizing quantitative resistance — which is expressed in plants at the adult stage as reduced development of necrotic tissue at the stem base compared with susceptible varieties — is another priority area for researchers at the U. of M., who are trying to determine how much quantitative resistance is present in current Canadian commercial varieties. “They found of all the varieties, 21 per cent would be considered to be resistant, so they have a high level of quantitative resistance and another quarter have a moderately resistant level. So half of those varieties have poor or low levels of quantitative resistance — again this is a potential concern for the industry,” says Jurke.
High levels of infection
Reports of high levels of blackleg infection in fields planted with R-rated varieties are also of utmost concern to the industry. The CCC and the research community are examining factors and collecting data and information about the circumstances surrounding incidences of resistance breakdown in the field. Not surprisingly, rotation plays a large role. Although in its early stages, a recent study indicates that of the data collected from one-third of the fields surveyed, approximately half are in a canola-on-canola rotation and the other half are in a two-year canola rotation, says Jurke.
Another factor considered in this survey of fields with high levels of blackleg planted with an R-rated variety is the previous canola variety planted in the field. So far, results indicate the majority of fields surveyed — 90 per cent — were not planted on the same variety, says Jurke.
Although information and data being collected in Canada will be used to help farmers make more informed decisions to manage blackleg, industry stakeholders are also considering systems being employed in other countries to fight the disease.
For example, of the top 25 selling varieties this past year in Canada, 22 fall into the resistant category and three in the moderately resistant. Jurke points out most varieties have similar labels and do not provide enough differentiation to enable producers to make informed decisions about the types of resistance that would be best applied to their farming circumstances. “Is this adequate information for the Canadian situation or do we need something different?” he asks.
Australia may prove to be a good role model for a new way of managing blackleg.
High levels of disease and yield losses prompted the country to create a unique system whereby resistance is grouped into categories (A, B, C, D, E, etc.). Canola varieties are assigned labels indicating the types of resistance genes they contain.
“Last year there was some indication that Group D-type of resistance was going to fail in an area of southern Australia. They put out a warning asking growers not to use this type of resistance in this geography and, essentially, what they predicted came to pass. The growers using Group E-type of resistance were fine — or A, B, or C — but when they used type D resistance they had epidemics in those fields,” says Jurke.
Presently, Ralph Lange, a researcher with Alberta Innovates Technology Futures, is assessing a similar protocol for Canada. In addition, these results support the validity of rotating types of resistance to stay ahead of blackleg disease.
Heading into the next growing season, there are a number of ways producers can help protect the resistant varieties currently available. A grower’s assessment of the risk potential for the development of blackleg in the field is an important step in the decision-making process, says Jurke. The CCC has devised a matrix to aid growers in the assessment of their blackleg infection risk in their fields and outlines the actions necessary to lessen that risk, says Jurke.
For example, when using the risk assessment tool, if growers determine they are in a high risk situation they can add diversity to their farming operations by extending crop rotations or rotating varieties, in addition to rotating fungicides if they are also in heavy use.
“If growers determine they have some high risk practices they have the opportunity to modify those practices or choose others that will help reduce that risk,” he says.
Keeping growers in the loop on new tools, information and developments on the blackleg front is also key to managing the disease and conserving existing varieties.
“We are working with the canola industry about what our next steps forward should be in identifying and describing blackleg resistance. If there are ways to provide more useful information to growers, hopefully we can develop a system that will provide meaningful labels for them,” says Jurke.